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B ∫ a F ( X ) F ( X ) + F ( a + B − X ) D X . - Mathematics

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Question

\[\int\limits_a^b \frac{f\left( x \right)}{f\left( x \right) + f\left( a + b - x \right)} dx .\]
Sum
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Solution

\[Let\ I = \int_a^b \frac{f\left( x \right)}{f\left( x \right) + f\left( a + b - x \right)} d x ................(1)\]
\[ = \int_a^b \frac{f\left( a + b - x \right)}{f\left( a + b - x \right) + f\left( a + b - a - b + x \right)} d x\]
\[ = \int_a^b \frac{f\left( a + b - x \right)}{f\left( a + b - x \right) + f\left( x \right)} d x\]
\[ \therefore I = \int_a^b \frac{f\left( a + b - x \right)}{f\left( x \right) + f\left( a + b - x \right)} d x ...............(2)\]
\[\text{Adding (1) and (2) we get}\]
\[2I = \int_a^b \left[ \frac{f\left( x \right)}{f\left( x \right) + f\left( a + b - x \right)} + \frac{f\left( a + b - x \right)}{f\left( x \right) + f\left( a + b - x \right)} \right] d x\]
\[ = \int_a^b \frac{f\left( x \right) + f\left( a + b - x \right)}{f\left( x \right) + f\left( a + b - x \right)} dx\]
\[ = \left[ x \right]_a^b \]
\[ = b - a\]
\[\text{Hence, }I = \frac{b - a}{2}\]

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Definite Integrals
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Chapter 20: Definite Integrals - Very Short Answers [Page 115]

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RD Sharma Mathematics [English] Class 12
Chapter 20 Definite Integrals
Very Short Answers | Q 20 | Page 115

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